Socket Size Chart: Metric and SAE (Inch) Conversions
Metric and SAE (inch) sockets almost never match exactly. A few pairs are true equivalents (8 mm and 5/16 in, 19 mm and 3/4 in, 27 mm and 1-1/16 in), a few are close, and some of the sizes you'll use most on a modern car, 10, 13, 14 and 17 mm, have no inch socket you should trust on a tight bolt. The chart below gives the exact conversions and says which swaps work, then explains which drive size suits which job.
What you need
- A metric socket set. Most modern cars, including US-built ones, use mostly metric fasteners. A 46-piece 1/4-inch socket and bit set covers 4 to 14 mm for small fasteners.
- SAE (inch) sockets if you work on older American vehicles, trailers or some machinery
- A caliper or a ruler with millimeter marks, to measure a head across the flats
- Your vehicle's service manual, which lists fastener sizes and torque specs
Metric and SAE socket size chart
Sizes are in order from smallest to largest. Conversions use the exact value of 25.4 mm per inch. The last column says whether the closest size from the other system is a safe substitute.
| Socket | Millimeters | Inches | Closest size in the other system | Swap? |
|---|---|---|---|---|
| 4 mm | 4.00 | 0.157 | 5/32 in (3.97 mm) | Yes |
| 5 mm | 5.00 | 0.197 | 3/16 in (4.76 mm) | No |
| 5.5 mm | 5.50 | 0.217 | 7/32 in (5.56 mm) | Yes |
| 6 mm | 6.00 | 0.236 | 1/4 in (6.35 mm) | No |
| 1/4 in | 6.35 | 0.250 | 6 mm | No |
| 7 mm | 7.00 | 0.276 | 9/32 in (7.14 mm) | Close |
| 5/16 in | 7.94 | 0.313 | 8 mm | Yes |
| 8 mm | 8.00 | 0.315 | 5/16 in (7.94 mm) | Yes |
| 9 mm | 9.00 | 0.354 | 11/32 in (8.73 mm) | No |
| 3/8 in | 9.53 | 0.375 | 10 mm | No |
| 10 mm | 10.00 | 0.394 | 3/8 in (9.53 mm) | No |
| 11 mm | 11.00 | 0.433 | 7/16 in (11.11 mm) | Close |
| 7/16 in | 11.11 | 0.438 | 11 mm | Close |
| 12 mm | 12.00 | 0.472 | No common inch size | No |
| 1/2 in | 12.70 | 0.500 | 13 mm | No |
| 13 mm | 13.00 | 0.512 | 1/2 in (12.70 mm) | No |
| 14 mm | 14.00 | 0.551 | 9/16 in (14.29 mm) | No |
| 9/16 in | 14.29 | 0.563 | 14 mm | No |
| 15 mm | 15.00 | 0.591 | 19/32 in (15.08 mm), a rare size | Yes |
| 5/8 in | 15.88 | 0.625 | 16 mm | Close |
| 16 mm | 16.00 | 0.630 | 5/8 in (15.88 mm) | Close |
| 17 mm | 17.00 | 0.669 | 11/16 in (17.46 mm) | No |
| 11/16 in | 17.46 | 0.688 | 17 mm | No |
| 18 mm | 18.00 | 0.709 | No common inch size | No |
| 19 mm | 19.00 | 0.748 | 3/4 in (19.05 mm) | Yes |
| 3/4 in | 19.05 | 0.750 | 19 mm | Yes |
| 13/16 in | 20.64 | 0.813 | 21 mm | No |
| 21 mm | 21.00 | 0.827 | 13/16 in (20.64 mm) | No |
| 22 mm | 22.00 | 0.866 | 7/8 in (22.23 mm) | No |
| 7/8 in | 22.23 | 0.875 | 22 mm | No |
| 15/16 in | 23.81 | 0.938 | 24 mm | No |
| 24 mm | 24.00 | 0.945 | 15/16 in (23.81 mm) | No |
| 1-1/16 in | 26.99 | 1.063 | 27 mm | Yes |
| 27 mm | 27.00 | 1.063 | 1-1/16 in (26.99 mm) | Yes |
How to read the Swap column
- Yes: less than 0.1 mm apart. The two sockets do the same job.
- Close: 0.1 to 0.15 mm apart. Fine on a clean, undamaged head at normal torque. On anything rusted, rounded or very tight, use the exact size.
- No: more than 0.15 mm apart. The smaller socket may not go on at all, and the larger one sits loose and pushes on the corners of the head, which is exactly how heads get rounded.
Spark plugs are a special case. Plugs listed as 5/8 in or 16 mm take either socket, and the large 20.8 mm plug hex is made to fit both 13/16 in and 21 mm spark plug sockets. Our guide to spark plug socket sizes covers this in detail.
Which socket fits which bolt?
A bolt is named by its thread, not its head. An M8 bolt has an 8 mm thread but usually needs a 13 mm socket. Head sizes also vary by standard:
| Thread | ISO head | Older DIN head | Japanese (JIS) head |
|---|---|---|---|
| M6 | 10 mm | 10 mm | 10 mm |
| M8 | 13 mm | 13 mm | 12 mm |
| M10 | 16 mm | 17 mm | 14 mm |
| M12 | 18 mm | 19 mm | 17 mm |
That is why the same M10 bolt might need a 14, 16 or 17 mm socket depending on the car, and why flange bolts and maker-specific bolts don't always follow the table. For standard inch hex bolts: a 1/4 in thread takes a 7/16 in socket, 5/16 takes 1/2, 3/8 takes 9/16, 7/16 takes 5/8 and 1/2 takes 3/4. Use these as a starting point and test-fit.
Socket drive sizes: 1/4, 3/8 and 1/2 inch
Drive size is the square hole in the back of the socket that fits the ratchet. It is always given in inches, even on metric sockets. It has nothing to do with the bolt size. It tells you how much force the tool is built to take.
- 1/4-inch drive: small fasteners at light to medium torque. Sockets usually stop around 13 to 14 mm. Interior trim, sensors, small brackets, engine covers, airbox clamps, license plates. The small ratchet head fits where bigger tools can't.
- 3/8-inch drive: the general-purpose size for most under-hood work. Sets typically run from around 6 mm to 19 or 22 mm.
- 1/2-inch drive: high-torque jobs such as wheel (lug) nuts, suspension bolts, hub and axle nuts, and crank pulley bolts. This is also the usual size for impact wrenches.
- 3/4-inch and 1-inch drive: trucks and heavy equipment.
Choose the drive by the force the job needs, not just by the socket size. A 10 mm bolt on an engine cover and a 17 mm bolt on a suspension arm need very different tools.
Where a 1/4-inch set fits in
A 1/4-inch set is the one you reach for on small jobs. The JUSMIR 46-piece set has a 24-tooth 1/4-inch ratchet, 13 metric sockets from 4 to 14 mm, 21 screwdriver bits including Torx T10 to T40, two extensions, a flexible extension, a sliding T-bar and a universal joint. That covers the 8, 10 and 13 mm heads on trim, covers and brackets. It is for light and medium torque only: wheel nuts and suspension bolts need a 1/2-inch drive tool. Its sockets are metric, so add inch sockets if you work on older SAE vehicles.
If you keep running into odd or worn heads on light jobs, a universal socket with spring-loaded pins takes the shape of many metric and inch heads (7 to 19 mm or 9 to 21 mm, depending on version). It is for everyday jobs, not seized or high-torque bolts, and not for impact wrenches.
Adapters and reducers
Adapters let you put a 3/8-inch socket on a 1/2-inch bar and so on. They're handy, but a 1/2 to 3/8 reducer lets a big breaker bar overload a smaller socket. Pull smoothly and stop if anything flexes.
6-point, 12-point, deep and impact sockets
- 6-point sockets grip the flats of a hex head and are less likely to round it. Use them on anything tight or rusty.
- 12-point sockets are easier to fit in tight spots and are needed for 12-point heads, but they grip nearer the corners.
- Deep sockets reach nuts on long studs and bolts that stick out.
- Impact sockets, usually black, are made to take the hammering of an impact wrench. Never use chrome hand sockets on an impact wrench: they can crack or shatter. Wear safety glasses either way.
How to find the right size fast
- Check the service manual or a parts catalog. It often lists the head size.
- Start with the most likely metric size and push the socket fully onto the head.
- Rock it gently. A correct socket has almost no play. If it wobbles, go down a size. If it won't start, go up.
- If you're unsure, measure across two opposite flats with a caliper and compare with the chart.
- On a seized or damaged head, use the exact size in a 6-point socket, never a "close enough" substitute.
Mistakes to avoid
- Using a 13 mm socket on a 1/2 in head, or a 9/16 in socket on a 14 mm head. Both are loose enough to round a tight bolt.
- Hammering a too-small socket on to make it fit. That is a removal trick for heads that are already rounded (see how to remove a rounded bolt or nut), not a way to work on good bolts.
- Putting a cheater pipe on a 1/4-inch ratchet. Step up to a bigger drive instead.
- Guessing torque. Tighten to the spec in your vehicle's manual with a torque wrench.
FAQ
What socket fits a 10 mm bolt?
A 10 mm socket. There's no inch equivalent: 3/8 in (9.53 mm) is too small to slide onto a good 10 mm head, and 7/16 in (11.11 mm) is far too loose.
Is 13 mm the same as 1/2 inch?
No. 1/2 in is 12.70 mm, 0.3 mm smaller. A 1/2 in socket usually won't go onto a good 13 mm head, and a 13 mm socket on a 1/2 in head is loose enough to round it.
Is 19 mm the same as 3/4 inch?
For practical purposes, yes. 3/4 in is 19.05 mm, only 0.05 mm bigger, so the two are interchangeable.
What size is 14 mm in standard?
The closest inch socket is 9/16 in (14.29 mm), which is about 0.3 mm loose on a 14 mm head. Use a real 14 mm on anything tight.
Which drive size should I buy first?
For general car work, a 3/8-inch metric set. Add a 1/4-inch set for small fasteners in tight spaces and a 1/2-inch breaker bar and sockets for wheels and suspension. Our list of basic car tools every driver should own covers the rest.
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